Don't follow the lights....
A Marsh Wraith attacking a Dunlending.
The Marsh Wraiths are mobs that will attack any mob (including the player) that are in the water of the Dead Marshes. You can not kill these mobs, and they will shoot projectiles at you endlessly until you are out of range. They can also be seen in the water being inactive when you look in. They will constantly disappear and reappear in a new location in the water you are looking in until you go or look away.